the spear is capable of killing pretty much anything. Spectre is just the upper limit, as is a 5D imp. That puts it at abstract killing power. But it also has the power to control the minds of super-powered beings.
Unfortunately, much like everything else Hitler was involved with or used personally, it's cursed with his evil. Anyone who uses it gets corrupted. Captain Marvel was able to resist it for awhile, but even he couldn't hold it.